1. Field of the Invention
This invention relates to a pneumatic fastener driving tool and more particularly to an improved valve means used in such a tool.
2. Background of the Invention
Pneumatic tools for driving fasteners such as nails, staples, brads and such are commonly used in the commercial work place. All of these devices have typical components comprising a housing to store compressed air, a cylinder in which a piston and driver combination is reciprocated therein, a valve means to provide pressurized air to the piston and a fastener carrier means to position successive fasteners underneath the driver prior to each driving stroke.
During operation of such tools, the tool is positioned in contact with the workpiece and the trigger is manually pulled which in turn operates a valve means that provides compressed air to the portion of the cylinder adjacent the top side of the piston. When the tool is used as a stationary device the trigger is replaced by a remote actuator.
As the piston reaches the end of the driving stroke, a return air chamber is pressurized to provide air pressure for the return stroke of the piston and driver. After the trigger is released, the valve closes blocking air into the top of the cylinder and in turn opens an exhaust port to release the air above the piston to the atmosphere. The stored air within the return chamber acts upon the underside of the piston to return it to the rest position at the uppermost portion of the cylinder.
To provide enough power to drive the fastener, the air must enter the cylinder above the piston quickly. To accomplish this, the valve means is normally divided into two functions. A first valve is located directly above the top of the cylinder and is shifted from a closed to open position pneumatically. By utilizing air pressure the valve can be held closed tightly and then opened with a snap action when air pressure on a portion of the valve is reduced.
To provide the change in air pressure on the first valve, a second smaller valve is actuated by pulling the trigger lever. The lever operates a plunger that in turn controls the movement of the main valve. This type of valve means is preferred since the force and movement needed is much less than that required if the main valve was moved directly by the trigger.
Most of the tools used for fastening applications are manually handled and used in environments where dirt and other contaminates exist that are detrimental to the life of the components. The parts most likely to cause failure are those associated with the valve. Although instructions are typically supplied with each tool informing users to regularly clean and lubricate the tool, many times it is not done or done properly resulting in damage to the components.
It is advantageous for this type of application to keep moving parts and air seals at a minimum. Most tools utilize O-rings as seals, but they require proper lubrication to provide long wear life. Other arrangements have been tried such as that disclosed in U.S. Pat. No. 4,747,338, however, this arrangement requires many additional components complicating the design. Also such a design subjects the seals to stretching due to unsupported sections when opposite sides of the seal are at a large difference in air pressure.
The seal design disclosed in U.S. Pat. No. 4,747,338 may be an improvement over O-rings, but component life can be further improved resulting is less need for service.